SUMMARY ? Project 3 Our goal is to improve the health and increase the lifespan of individuals affected with two devastating cancers, acute myeloid leukemia (AML) and glioblastoma multiforme (GBM). Patients diagnosed with either disease typically survive for less than two years. These tumors initiate in different tissues, but they share features that motivate our focus on both in a single Project and within a single Program. These include genetic instability, epigenetic dysregulation, and an actively evolving cell population that contributes to the intratumor heterogeneity that enables emergent subclones to thwart initial therapy. The central hypothesis of our proposed research is that the response to therapy can be improved by targeting the mechanisms that promote intratumor heterogeneity. Our specific aims are: 1. To apply an innovative mechanism-based assay developed in the last funding period to quantify the signature protein-DNA adducts produced by decitabine (5-aza-dC), commonly used to treat AML, in order to enable individualized therapy and to discover new and more specific drugs. 2. To improve radiation therapy of GBM, by identifying markers that correlate with radiation resistance and by establishing how treatment with radiation affects genomic instability that enables tumor cells to thwart therapy. 3. To establish the utility of mechanisms that promote intratumor heterogeneity as drug targets in two contexts, by determining the functional consequences of recurrent IDH1/2 mutations and of subclonal mutations in the POLD1 and POLE genes that encode replicative polymerases ? and ?. The results of this research will provide new understanding of the mechanisms of intratumor heterogeneity and have considerable potential to impact the design and implementation of current therapeutic regimens.